Production of ozone by electrolysis of water has hitherto been carried out, and high purity ozone in a high concentration can be obtained by the following two methods.
The first method comprises electrolysis of a solution containing a highly electronegative anion as an auxiliary electrolyte. The method is referred to as solution electrolysis. The second method comprises electrolysis of water using a high polymeric solid electrolyte. Although the former method achieves an extremely high current efficiency with proper selection of an electrode substance, a solution (electrolytic solution), electrolytic conditions, etc., the method is still under study in laboratories and has not yet been embodied into a practical and commercial apparatus because the electrolytic solution is very corrosive.
The latter method is the so-called solid electrolyte type or zero gap type electrolysis in which a perfluorocarbon sulfonic acid type cation exchange membrane is used as a solid electrolyte intimately sandwiched in between a cathode and an anode. This method involves no corrosive or dangerous substance except that which is ozone generated, and the apparatus therefor is relatively simple in structure and easy to handle. For these reasons, several types of apparatus have been commercialized.
The current efficiency in ozone generation achieved by the conventional apparatus is usually from 13 to 18% and about 20% at the highest, and the resulting gas is water-saturated oxygen gas containing 13 to 18 wt % of ozone. Since the liquid component in this electrolytic system is deionized water that is regarded to be only slightly corrosive, the system does not entail wear of the electrodes, elution of other components, or incorporation of impurities, thereby providing a nearly pure, mixed gas.
Use of ozone, which has formerly been directed to sterilization, has recently been extending over the fields of precision engineering, such as washing of electronic parts. In this connection, the above-described solid electrolyte type ozone generator is disadvantageous in that it requires higher electric power than discharge type ozonizers. Depending on the discharge method, the power consumption usually ranges from 15 to 20 Wh/g-ozone (ozone concentration: 2 to 3%). In order to increase the ozone concentration obtained by the discharge method to about 10%, it is necessary to equip the apparatus with a cooling means and to replace air with oxygen as a starting gas. It is said that the power consumption under the thus altered conditions would be from 70 to 80 Wh/g-ozone. On the other hand, the method using a solid electrolyte, while always attaining an ozone concentration as high as 13 to 18%, requires a power consumption of 70 to 80 Wh/g-ozone, which has been a great problem waiting for a solution.
The causes of the high power consumption include, for example, the lower current efficiency (13 to 18%) in ozone generation by water electrolysis compared with ordinary electrolysis and involvement of evolution of hydrogen unnecessary for ozone production, which also consumes power. In addition, the by-produced hydrogen gas diffuses into the resulting ozone-containing gas in a proportion of from 0.1 to several percents.
Attempts to improve the current efficiency, and thereby to reduce the power consumption, have continued up to date. However, a means for carrying out ozone generation in a stable manner at a markedly improved current efficiency has not yet been developed.
As an approach for reducing the power consumption through control of hydrogen by-production, it has been proposed to use an oxygen gas cathode as reported in Soda Kogyo Gijutsu Toronkai Yoshishu (1992). In this method, the cathodic reaction accompanied by hydrogen evolution, 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2, is converted to one involving no hydrogen evolution, H.sub.2 O+1/2O.sub.2 +2e.sup.- .fwdarw.2OH.sup.-, whereby the power consumption can be reduced by that assigned to hydrogen evolution. In fact, the voltage of from 3.1 to 3.6 V that is required when an oxygen gas cathode is not used can be reduced by 0.5 to 0.6 V. However, because the electrolysis according to this method is carried out at such a high current density of 50 A/dm.sup.2 or more at a low temperature of 40.degree. C. or lower, the overpotential inevitably becomes higher, resulting in an increase of heat generation. That is, the use of an oxygen gas cathode decreases the voltage but, in turn, necessitates a separate cooler or chiller. As a result, the overall reduction in energy seems to be very slight. Therefore, it has been necessary to develop a method for accomplishing efficient reduction in voltage.